Hydrostatic transmission control system



Sept. 1, 1970 J. R. CRYDER ET AL 3,526,283

HYDROSTATIC TRANSMISSION CONTROL SYSTEM Original Filed June 14, 1967 1OSheets-Sheet 1 vENGINE Sept. 1, 1970 J. R. CRYDER ET 3,526,283

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Sept. 1, 1970' Original Filed June 14, 1967 STEERING PRESSURE (PSI) J.R. CRYDER ET AL (526,288

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Sept. 1, 1970 J, R, CRYDER ET AL 3,526,288

HYDROSTATIC TRANSMISSION CONTROL SYSTEM Original Filed June 14, 1967 10Sheets-Sheet 10 To LINE 76 United States Patent 3,526,288 HYDROSTATICTRANSMISSION CONTROL SYSTEM John R. Cryder, Joliet, Willard J. Haak,John L. Hufeld, and Lionel L. Kinney, Peoria, Kenneth R. Lohbauer,Joliet, and Howard A. Marsden, Pekin, Ill., Ralph W. Matthews, Franklin,Wis., and William B. 'Norick, Joliet, Glen E. Stewart, Springfield,Rollin P. Vanzaudt, Peoria, and Frank H. Winters, Joliet, 111.,assignors to Caterpillar Tractor Co., Peoria, 111., a corporation ofCalifornia Original application June 14, 1967, Ser. No. 645,912, nowPatent No. 3,477,225, dated Nov. 20, 1969. Divided and this applicationAug. 2, 1968, Ser. No. 763,462 Int. Cl. B62d 11/04 U.S. Cl. ISO-6.48 3Claims ABSTRACT OF THE DISCLOSURE Hydrostatic transmission systems,which transmit torque via fluid pressure circulating between a pump andmotor, are now sufiiciently perfected for use in work vehicles.Infinitely variable input to output ratios are available in suchtransmissions which make them espe cially useful in tractor typevehicles. However, for high efliciency the transmission ratio must beproperly and continuously adjusted for maximum performance. Such controlis accomplished in this invention by employing a separate control pumpgeared to the transmission power source, a valve controlleddiflferential pressure stack connected to receive the output of thecontrol pump wherein changes in its valve position will eifect changesin pressure differential ocurring within the stack, pressure responsivemeans having elements displaceable by pressure differential connected tothe stack so that pressure diiferentials occurring therein arecommunicated to the elements and linkages connecting the elements withcontrol means in the transmission for changing its ratio. Also includedis a flow sensitive system which is connected across the diiferentialpressure stack to reduce the pressure differential if the output of thecontrol pump drops due to a decrease in engine speed. The basic controlsystem can be employed with a variety of subsystems to provide steeringin tracktype vehicles, reversing and the like.

This is a divisional application of U.S. application Ser. No. 645,912,filed June 14, 1967, now Pat. No. 3,477,225.

BACKGROUND OF THE INVENTION Advances in the technology of hydraulictranslating devices (hydraulic pumps and motors) now allow hydrostatictransmissions to be employed in vehicles having high drawbar horsepowerrequirements. With its infinitely variable speed ratio between theengine and the ground speed of the vehicle the hydrostatic transmissionotters the ability to obtain the maximum drawbar horsepower over thevehicles full speed range. An early hydrostatic tarnsmission for atrack-type vehicle is disclosed in U.S. Pat. No. 2,036,437 issued toRuediger, but at that time the technology of translating units was notperfected suiticiently to bring it to reality.

With the new translating devices now available, the limited gear stepsin work vehicles, associated clutching, shifting, braking and engineacceleration and deceleration, may become a thing of the past.Translating devices suitable for hydrostatic transmissions in workvehicles are disclosed in copending application Serial No. 564,875entitled Hydrostatic Apparatus, now U.S. Pat. No. 3,388,472, whereinmany of the advantages of hydrostatic transmissions are noted.

3,526,288 Patented Sept. 1, 1970 For a hydrostatic transmission to havea broad speed range without large pump and motor units, it is necessarythat the displacement of both its pump and motor be varied indisplacement. This avoids the need for both high and low speed motors,such as disclosed in U.S. Pat. 2,541,290 issued to Robinson, or undulylarge translating units to obtain the desirable speed ranges.

Hydrostatic transmissions in which both the pump and motor are variabledisplacement units present a speical probelm in control since thechanges in displacement of the pump and motor relative to one anothershould be properly sequenced for eflicient operation and to providedesirable torque ratio characteristics in the transmission. Ina properlysequenced transmission, the pump will have zero displacement at a zerospeed condition, and the motor will be at its maximum displacement. Foracceleration of the vehicle, the pump is increased in displacementtoward a maximum displacement, while the motor remains at its maximumdisplacement so it will develop maximum torque at minimum presures foraccelerating the vehicle.

After the pump reaches its maximum displacement, the vehicle is atmaximum speed unless the displacement of the motor is variable. Theneven with small size translating units, a broad speed range is obtainedby decreasing the displacement of the motor to further increase thespeed of the vehicle with such a transmission.

Also, these transmissions in a vehicle should be properly sequenced froma high speed to a lower speed, somewhat similar to downshifting aconventional transmission. Thus, for a speed decrease from maximumtransmission speed, the motor displacement is first increased to itsmaximum to initially slow the vehicle and thereafter the pump is movedtoward its zero displacement to further retard vehicle speed.

If proper sequencing is not employed desirable torque ratiocharacteristics are lost with an accompanying decrease in transmissionefliciency; operation of the transmission may become rough in somemodes; retarding (braking) action through the transmission is reducedduring deceleration; and unnecessary high pressures will be developed inthe transmission.

Some control systems for hydrostatic transmissions have been designedwhich sense the pressure in the fluid loop connecting the pump and motorand use this pressure to control the transmission. However, those typesof control systems are not satisfactory since, in many cases, the enginedriving the transmission must also power mounted or towed implementswhich make power demands on the engine, without changing the pressure inthe hydrostatic loop. Further, loop pressure can vary independent ofengine load.

Accordingly, it is a purpose of this invention to provide a simple butreliable control system for hydrostatic transmissions which is capableof sensing the total load on an engine and automatically adjusting ahydrostatic transmission for maximum output in relation to the poweravailable.

Another purpose of this invention is to provide a control system forpaired hydrostatic transmissions in tracktype vehicles wherein thetransmissions can be independntly controlled for steering the vehicles.

Also, it is a purpose of the invention to provide complementarycomponents for the basic control system which increase its flexibilityand improve its performance.

SUMMA RY Briefly, the above purposes and advantages as well as manyothers, can be accomplished in a basic control system for a hydrostatictransmission comprising a positive displacement control pump geared tothe prime mover driving the transmission, a venturi means connected tothe pump so the output of the pump passes therethrough,

a differential pressure control stack also connected so the output ofsaid control pump passes therethrough, a manually adjusted valve in saiddifferential pressure stack for controlling the differential pressure inthe stack, biased valve means associated with the venturi means andconnected across said differential pressure stack operable to reduce thedifferential pressure in the stack when the engine speed decreases underload, pressure reciprocated pilot means connected to said stack andhaving a movable element therein which changes position in response tothe differential pressure in the stack, and linking means connectingsaid element to the servo-units in the transmission for controllingdisplacement of translating units, and thus, transmission ratio.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 diagrammatically illustratesthe basic control system associated with a hydrostatic transmission;

FIG. 2 illustrates a known relief and replenishing group plus a highpressure relief system for the hydrostatic transmission shown in FIG. 1;

FIG. 3 diagrammatically illustrates a modification to the basic controlsystem shown in FIG. 1 which provides variable engine speed compensatingmeans for the control system and linear servo systems for transmissionratio control, along with sequence system valves for forward-reverseshift;

FIG. 4 is a block diagram showing the relationship between FIGS. 5 and6;

FIG. 5 shows part of the control system when employed for dualhydrostatic transmission systems in track-type vehicles with provisionsfor steering;

FIG. 6 discloses the complementary parts of the control system shown inFIG. 5 and also the transmission units;

FIG. 7 shows the dual transmissions shown in FIG. 6 mounted in atrack-type vehicle;

FIG. 8 illustrates a relief and replenishing valve unit Which isemployed with the integrated control system shown in FIGS. 5 and 6,providing additional control features in the system;

FIG. 9 illustrates a safety valve system which cooperates with therelief and replenishing valve shown in FIG. 8 to deactivate thetransmission for operator safety;

FIG. 10 diagrammatically illustrates a steering control system employedwith track-type vehicles using dual transmissions as shown in FIGS. 5and 6;

FIG. 11 shows schematically a hydraulic circuit for the steering controlsystem of FIG. 10;

FIG. 12 graphically shows the proportion between control movement andresponse of the steering circuit; and

FIG. 13 illustrates a variable orifice for flow device Which can be usedin place of simple ball checks and parallel orifice units in moresophisticated control systems for smooth transmission response.

DESCRIPTION OF EMBODIMENTS In understanding the control system it ishelpful to associate it with a hydrostatic transmission. Therefore, FIG.1 illustrates the control system with a hydrostatic transmission 30having a variable displacement pump 31 and motor 32 connected togetherin a hydraulic loop circuit composed of an upper conduit 33 and a lowerconduit 34. Upper and lower with reference to the conduits are merelyfor convenience and not indicative of position or structure.

The transmission illustrated in the drawings uses variable displacementtranslating devices which are illustrated in copending application Ser.No. 564,875 entitled Hydrostatic Apparatus having a common assignee.These particular translating units are suitable for high-pressureoperation since the hydrostatic loop is completely rigid with theconduits 33 and 34 joined directly to rigid trunions which extendcompletely through these units. This forms a very rigid transmissionstructure.

For actuation of the transmission the units are swung about theirtrunnion to change the displacement. Pump actuator 35 through its rod35a swings the pump for changes in displacement with motor actuator 36swinging the motor about its trunnion via rod 36a. The pump actuator iscontrolled by the pump rotary servo unit 37, while the motor actuator isindependently controlled by a separate motor rotary servo 38. Theseservo units are mounted adjacent to the pump or motor which they arecontrolling so that follow-up linkages 37a and 38a can be connected tothem for controlling position correspondence in the servo units.Hydraulic lines 39 connect the respective actuators with theirassociated rotary servo.

The rotary servo valve units can be of a conventional type, such asshown and described in the U8. Pat. No. 1,773,794 issued to Schneider,and control the flow of pressurized fluid from line 40 to theirassociated actuator in order to change the pump or motor displacement. A.pump servo control arm 41 positons the pump servo, which in turn allowsfluid to flow to actuator 35 until the pump position has swungsufficiently that follow-up bracket 37a repositions parts of the servoto stop the flow of fluid when position correspondence is achieved.During movement of the actuator fluid is drained from the unpressurizedend of the actuator to reservoir via the servo unit and drain 43.

Motor rotary servo unit 38 operates in the same manner, thus motor servocontrol arm 44 will change displacement of the motor by allowingpressurized fluid to flow from line 40 to the actuator 36 via line 39until such time as follow-up bracket 38a has repositioned the servo unitto stop fluid flow at position correspondence. Fluid from theunpressurized side of the motor actuator is drained to reservoir via theservo and drain 46.

It should be appreciated that the particular translating unitsillustrated are not intended to limit the invention, and that othertypes of fluid translating units, such as those shown in US. Pat.3,274,946 issued to E. E. Simmons, could be employed and controlled bythe servo control units associated with the control system.

In most hydrostatic transmissions leakage is not sufficient to allowenough make-up fluid to be added to the fluid loop for cooling purposes.Thus it is necessary to utilize a relief and a replenishing group 47(enclosed in broken lines), such as shown in FIG. 2, so some of thefluid can be removed from the loop and make-up fluid add-ed for coolingthe transmission. Operation of such a group is discussed in order that afully integrated control system according to this invention may beunderstood.

In FIG. 2 a prior art relief and replenishing group 47 with ahigh-pressure relief is illustrated. In a hydrostatic transmissioneither the upper or lower conduit 33 or 34 may be operated at a higherpressure, as the working side of the loop, depending upon the power flowthrough the transmission. Thus the relief and replenishing group must beable to remove and add hydraulic fluid from either side of the loop(conduit 33 or conduit 34). Typically this is accomplished by twoseparate sections in the relief and replenishing group, which, forconvenience, can be designated as replenishing section 51 and the fluidrelief section 52 (outlined with broken lines).

In the replenishing section two spring-biased ball checks 53 and 54 arein communication with conduits 33 and 34, respectively, through stubpipes 53a and 54a. These checks, illustrative of type of checks whichcan be employed, are arranged so that pressurized fluid from line 40 canpass through one or the other check valve into the hydraulic loop,providing a positive pressure in the conduit operating at the lowerpressure, preventing pump cavitation and adding cool fluid to the loop.The higher pressure in the conduit operating as the working side of theloop will close its associated ball check so that power will not be lostby flow of pressurized fluid therethrough.

The replenishing fluid from line 40 is precooled in order to providetransmission cooling, and the fluid relief section 52 is used toremove'fluid from the loop so that a greater amount of cool replenishingfluid can be added through ball check valves. Hydraulic fluid in theloop is removed through the relief fluid dump valve 55 subsequent tocirculating through the motor. The dump valve is connected between theconduits 33 and 34 with short pipes 55a. A spring centered spool 56 inthis valve is arranged so its opposite ends are in communication withthe pressurized fluid in conduits 33 and 34 via the stub pipes. Normallythis spring centered spool will remain in a neutral positon until eitherconduit 33 or 34 operates at a higher pressure, at which time it will bedisplaced in a direction to open a port 57 communicating with theconduit operating at the lower pressure. As this occurs a reduced centerportion 56b will open a flow passage through one of the tap conduits 58on the main loop, through chamber 550, conduit 59, relief valve 60 andcooler 61 reservoir. Opening this path allows additional fluid to beremoved from the loop subsequent to passing through the motor so that agreater amount of replenishing fluid can be introduced through one ofthe ball check valve-s, as previously noted.

Also, it is desriable to incorporate an overpressure relief valve system62 in the loop of a hydrostatic transmission. In FIG. 2 such a system isconnected between conduits 33 and 34 with short connecting conduits 62a.These short conduits communicate with separate chambers which are closedby a spring-loaded poppet valve 63 whose bias is adjusted to control themaximum pressure in the loop. The chamber behind each poppet isconnected with the short conduit 64 with the chamber closed by theopposite poppet so that when over-pressure occurs displacing one of thepoppets, fluid can vent from one side of the loop directly to theopposite side, bypassing the motor. A ball check 64a in conduit 64prevents reverse flow in each line.

The above description covers a typical hydrostatic transmission and itscomponents with which the novel control system of this invention isemployed. From the above discussion, it can be appreciated that thecontrol system must cooperate with the servo-control units of the pumpand motor in order to achieve proper sequencing of the transmission.

Normally such a transmission as described above will be directly drivenby an engine 65, through shaft 66, as illustrated in FIG. 1. It is notnecessary, though optional, to have a clutch between the engine and the.pump since the pump can be positioned for zero displacement toneutralize the power flow in the transmission without disconnecting thepump. The engine also drives the implement pump 67 through shaft '68,which also makes power demands on the engine during operation ofvehicle-mounted or towed implements.

The control system of this invention is designed to properly sequence ahydrostatic transmission, like the one described above, under allconditions of load. It automatically adjusts the transmission for themaximum power output consistent with power available from the enginewithin a manually set speed range.

As previously noted, the control system includes a positive displacementpump geared to the engine 65, a venturi means connected to receive theflow of the pump, a difl erential pressure stack also connected toreceive the flow of the pump, a manually adjusted valve in thedifferential pressure stack for controlling the pressure dilferential,biased valve means associated with the venturi connected into the stackwhich also controls the differential pressure therein in response tochanges in engine speed, pressure reciprocated pilot means connectedacross the stack and having an element movable therein which changesposition in response to differential pressure, and linking meansconnecting said elements to servo-valve units for controlling theactuators in the transmission.

More specifically, the control pump 70 is connected to the enginethrough its shaft 69 and is of the positive displacement type. Thus, theoutput of the control pump will be directly proportional to the speed ofengine 65.

Pressurized fluid from the pump flows via conduit 71 through venturi 72and conduit 73 to the differential pressure slack 74. The differentialpressure stack can be fabricated in a number of Ways, but the simplestis the use of two parallel tubes 75 and 76, with tube 75 connecteddirectly to the output of the venturi and a manual valve leading from itto tube 76. A conduit 77 carries the fluid from tube 76 of the stack toreservoir via relief valve 78, or through pressurized line 40 to therelief and replenishing group 47 of the transmission (previouslydescribed) from which it is eventually returned to reservoir.

Pressurized fluid from tube 75 passing to tube 76 of the differentialpressure stack must pass through the manually-operated speed controlvalve 79, or an underspeed valve 80, the latter normally being closedwhen the engine is operating at rated speed. The speed control valveincludes a modulating valve spool 81 which is positioned by themanually-manipulated speed lever 82 through linkages 83. Movement ofthis spool restricts or opens the passage between the tubes of thedifferential pressure stack. As the restriction increases the pressurein tube 75 will increase above that of tube 76, since the pump flowvolume will remain constant at a selected engine speed. Thus, thegreater the restriction of the passage by the spool, the greaterpressure differential there will be between the two tubes of the stack.

The location of the venturi in the control system is not critical solong as all the control pump flow passes therethrough. It can be locatedeither upstream or downstream of the differential stack since it isresponsive to the flow rate from the control pump and cooperates withthe underspeed valve to open a parallel path to that controlled by thespeed control valve when the engine speed drops off and control pumpoutput decreases.

Basically, underspeed valve 80 is designed to function when the enginefalls below its rated speed for which the control system is designedwhich results in a decrease in output from the control pump 70. Whenthis occurs the pressure in the low-pressure tap 72a of the venturi willincrease causing a greater pressure to be reflected in a chamber behinda valve spool 80a containing the spring bias in the underspeed valve.Thus, the pressure from the low-pressure tap of the venturi and thespring bias overcomes the pressure in the inlet throat of the venturivia line 71a reflected on the opposite side of the spool. As long asthis pressure exceeds that in the low pressure venturi tap and thespring 84, the valve spool will be urged into the spring to bottom on ashoulder in the valve body to close off this passage between the tubesof the stack through the underspeed valve. Therefore, decreased outputof the control pump due to a reduction in engine r.p.m. will graduallyopen a flow path between the tubes through the underspeed valve causingthe previously established pressure dilferential in the stack todecrease in proportion to the drop in engine r.p.m. thereby reducingtransmission torque requirements until the engine can maintain ratedr.p.m. Normally this underspeed valve is set for actuation when theengine speed drops from 3 to 10%, but can be adjusted for a desiredsensitivity.

From the above description it should be appreciated that the aboveelements of the control system can create a pressure dilferential in twotubes of the stack which is reduced automatically when an underspeedcondition exists by the opening of the path through the underspeedvalve. By interconnecting pressure responsive means which have elementsthat move in response to changes in the differential pressure, it ispossible to utilize the system to control a hydrostatic transmission.Normally these pressure sensitive elements are connected to servocontrolmeans since they are not generally capable of generating the forcesnecessary to change the ratio of the transmission.

Typical of such pressure sensitive elements is pump pilot cylinder whichcontrols servo-control arm 41 of the pump servo 37 by a connection tothe arm through adjustable linkages 91. Its piston element 92 iscentered in the pilot cylinder by springs 93, and the opposite ends ofthe pilot cylinder are in communication with opposite tubes of thedifferential pressure stack 74 so that any pressure differential in thestack Will be reflected on opposite sides of the piston element.Obviously, some preload to the centered position is desirable to insureaccurate positioning at zero differential.

Connected in this manner, any pressure differential existing in thestack will cause the piston of the pilot cylinder to be displaced in adirection sufficient to balance the differential against increasingspring bias. It should be appreciated that the pilot cylinder operateson pressure differential and thus absolute pressures in the system arenot critical.

Since the pump unit is the reversing unit of the transmission andreverse the power flow (direction of fluid flow), it is necessary toprovide means for reversing the connection of lines 94 and 95 connectingthe opposite ends of the pump pilot cylinder to the differential stackto enable its pilot cylinder to operate in both forward and reverse.Forward-reverse valve 96 is used for this purpose, and can switch theconnection of line 94 from tube 75 of the differential stack to tube 76of the differential stack for reversing. Line 95 is simultaneouslyswitched from tube 76 to tube 75, which allows the linkage connectingthe piston element to the servo-control arm 41 to properly position thepump for the reverse power flow.

Since the pump pilot cylinder tends to react rapidly to pressure changesin the differential pressure stack, transmission control may be roughunless means are provided to smooth out the rate of movement of thepiston element in the pump pilot cylinder. This is accomplished by aparallel ball check and orifice device 97 between the forward andreverse valve and the tubes of the differential pressure stack, as shownin FIG. 1. Since tubes 75 of the differential pressure stack will alwaysbe operating at the high pressure when the transmission is running theball check in the line connecting this tube to the forwardreverse valveis oriented to meter the flow to the valve. This limits the rate ofpressure change in the pilot cylinder and thus smooths out movement ofits piston element. A similar device incorporated in the connectionbetween valve 96 and tube 76 of the differential stack is reversed sothat flow is unrestricted by the ball check from this tube toward thepilot cylinder, but is restricted from the cylinder toward tube 76.Through the above arrangement, the piston in the pilot cylinder willreact smoothly for good transmission response.

In the above control system, it should be appreciated that there isessentially no net fluid flow required to operate the system since itresponds to differential pressure and movement of the pilot cylinderwill return the same amount of fluid from one side as is being receivedby the other so that the net flow is always zero. Thus, actuation of thecontrol system does not result in any pressure fluctuation due to fluidflow demands.

As pointed out above, the transmission is reversed by reversing thepower flow (fluid flow) through the pump, and thus the motor 32 willautomatically reverse with the ".change in direction of fluid flow.Thus, motor pilot cylinder 100 is connected directly to the differentialpressure stack without a forward-reversing valve. However, the lines 101and 102 connecting the motor pilot cylinder to the differential pressurestack contain parallel ball check and orifice devices 97 to smooth outthe actuation of the motor pilot cylinder in the same manner asdescribed for the pump pilot cylinder. Other modulating means may beemployed in place of the ball checks and orifice devices.

As previously noted, the proper sequencing of the transmission requiresthat the pump 31 be at a maximum displacement in either direction beforethe displacement of the motor 32 is decreased for increasing the speedof the transmission. This is accomplished in the control system of thisinvention by a difference in construction in the motor pilot cylinder.Basically, the motor pilot cylinder has a single spring 103 which biasesits piston 104 to one end of the cylinder against a stop 104a. The sideof the piston without the spring is in communication through line 101with tube of the differential pressure stack, while the biased side ofthe piston is in communication with tube 76 of the differential pressurestack via line 102. The motor pilot cylinder has an adjustable linkage105 connecting its piston to the control arm 44 of the motor servo unit38.

Using the above construction and proper selection of the spring biasingunits in the pump pilot cylinder and motor pilot cylinder, propercontrol of the transmission can be accomplished by selecting the bias inthe motor pilot cylinder so the piston of the pump pilot cylinder willbe displaced a maximum in either a forward or reverse direction at alower pressure differential than is necessary to effect movement of thepiston in the motor pilot cylinder against its large spring bias. Thus,as differential pressure in the stack increases, the pressure reflectedon the piston of the motor pilot through line 101 will eventuallyincrease sufficiently to move piston 104 against the spring bias andthrough the linkage with the servocontrol arm effect an adjustment ofthe motor for an increase in speed, after maximum pump displacement.

Through the unique arrangement described above, the transmission willalways be properly sequenced by the control system. For example, as thedifferential pressure in the stack increases from zero, the pump pilotcylinder will be first to react and will be displaced to a maximum priorto the actuation of the motor pilot cylinder which requires a slightlyhigher pressure for it to react. Thus, proper sequencing from low tohigh speed and from high to low speed in the transmisison will always beachieved. Also, during all phases of sequencing this novel system, thetransmission is automatically adjusted for the power available since thecontrol pump 70 will always reflect engine speed and any decrease inspeed will automatically change the differential pressure in the stackthrough the action of the underspeed valve until the engine can maintainits rated speed, or a close approximation thereof. Therefore, anyhorsepower demands from the implement pump 67 or other engine-connectedunits will not lug down the engine since the transmission willautomatically reduce vehicle speed until the engine is able to hold itsrated speed. This means that the implement circuits will operate morerapidly (higher pump r.p.m.) and that the maximum efliciency as a unitcan be obtained from the vehicle using this control system with ahydrostatic transmission.

The basic control system shown in FIG. 1 is designed for anenginerunning continuously at its rated speed, and in order to provide thebasic control system with greater flexibility so that an operator canselect various engine speeds below rated, a modified system is shown inFIG. 3. Also, in this design the actuators for controlling thedisplacement of the pump and motor and the servo units are shown as alinear system. Other than these changes the system is very similar tothat shown in FIG. 1 and identical parts are similarly numbered.

In this modified system, to operate the transmission control systembelow a rated rpm, the underspeed valve must be modified since otherwiseit would always sense an underspeed condition if constructed as shown inFIG. 1, and prevent the transmission from operating below engine ratedspeed. A modified underspeed valve is connected across the differentialpressure stack 74 so that its control spool 111 can open a path parallelto that controlled by the speed control valve 79, as previouslydescribed. However, the spool is connected to a speed pilot cylinder 112whose piston 113 is springcentered in the cylinder. This piston has oneside in communication with the low-pressure outlet 72a of the venturi72, and the other side in communication with the inlet of the venturithrough line 114. Thus, changes of flow through the venturi will causethe piston to move from a first equilibrium position to a newequilibrium position, moving the spool of the underspeed valve throughthe linkage. Since any equilibrium condition in the speed pilot cylinderis for a particular speed (flow through the venturi), the bias on thecylinder must be changed for any different engine speed selected so thatthe underspeed valve will function properly at the selected enginespeed. This change in bias is accomplished by spring 115 biasing thespool according to the position of cam 116 connected by linkage 117 tothrottle 118. The throttle also acts the governor 119 of the engine andthrough this arrangement the spring bias on the speed pilot cylinder isadjusted so that the underspeed valve will operate at the speed selectedby the throttle rather than at a single preset speed. Through thisarrangement the speed control unit can be used for selected enginespeeds.

Also, in FIG. 3, the actuator and servo systems are shown as simplelinear systems, but the pump pilot cylinder 90 and the motor pilotcylinder 100 are the same and connected to the pressure stack in thesame way as those shown in FIG. 1 except the parallel ball check andorifice units 97 are not shown in the lines of the motor pilot cylinder.

Since the operation of the pilot cylinders have been described, it isonly necessary to describe the operation of the changed actuator system,shown in FIG. 3 for controlling the transmission. Basically, the pilotcylinders are connected through adjustable links 120, servo spools 121in their respective actuators 122 and 123. Movement of the servo spoolswill open one of the ports 124 communicating between the actuator andthe servo so that pressurized fluid can flow from line 40 to theactuator causing the housing to move in a direction which will close theport repositioning the housing relative to the servo spool to close theport. The rods 125 of the actuator pistons 126 are secured to the frameand by connecting the housing of the actuator 122 through adjustablelinkage 127 to the pump and likewise connecting the housing of actuator123 through linkage 128 to the motor of the transmission, displacementof the pump and motor can be controlled by the pilot cylinders in themanner previously indicated. The adjustable linkages connecting theactuator housings to the translating units may be used to properly nullthe transmission for zero speed.

When operating at higher transmission speeds, where the displacement ofthe motor has been reduced to increase speed, the forward-reverse shiftaccomplished by valve 96 may not sequence the transmission properly forthe best torque ratio characteristics, if the motor pilot cylinder 100is connected as shown in FIG. 1 in FIG. 3, proper sequencing during aforward-reverse shift at high speed is insured by connecting line 101 ofthe motor pilot cylinder to the side of the pump pilot cylinder 90,which is at the higher pressure, through a shuttle unit 129.

The spool 129a of this unit has a center land and two smaller outboardlands with grooves in between, so that when the land is centered, line101 is closed off. When the spool is offset to one side, line 101 willbe in communication with one of the grooves. One groove is incommunication with line 95 of the pump pilot cylinder, while the otheris connected with line 94. Thus, depending on the direction of thedisplacement of the spool due to the differential area between thecenter land and the smaller lands at the ends of the spool, one of thepump pilot lines (the line at higher pressure) will be in communicationwith line 101 of the pump pressure in the motor pilot cylinder. Sincethe lands at opposite ends of the spool are smaller than the centerland, the line operating at the higher pressure will displace the spoolso that it establishes communication with line 101 of the motor pilotcylinder.

Connected in this manner, when a forward-reverse shift is accomplishedby moving valve 96, the higher pressure sides of both the pump and motorpilot cylinders will flow back to the forward and reverse valve, andthence through the restricting ball check and orifice device 97. Sincethe motor pilot cylinder requires a much greater pressure to displaceit, it will first bleed through the system, holding the pump pilotcylinder at its maximum displacement until it has bled down to bottomout. At this time, the pump pilot cylinder will start to bleed down inorder to accomplish a reverse shift. As this has occurred, the higherpressure in the opposite line will begin to reverse the pump pilotcylinder for a reverse power flow, and the spool 129a will shift sofluid pressure will continue to displace the element of the pump pilotcylinder until the pump reaches maximum displacement. In this way, theconnection of the motor pilot cylinder is shuttled between lines 94 and95 of the pump pilot cylinder so that proper sequencing is alwaysinsured during a forward-reverse shift.

The center land will close off fluid communication of line 101 with theside of the pump pilot cylinder, changing from the lower pressure to thehigher during the forward-reverse shift, until the motor pilot has movedfor maximum displacement and the pump pilot has moved for zerodisplacement, at which time the spool shifts.

Grooves 12911 at each end of the shuttle unit are closed by the smalllands when the spool is in neutral and communicate with the low pressureside of the stack. Before the spool shifts during a forward-reverseshift, the high pressure from the forward-reverse valve 96 will godirectly to the low pressure side of the stack until the pump pilotelement moves to zero displacement; at this time, the spool shifts andpressure buildup occurs in both the pump'and motor pilots to reverse thetransmission. This prevents the higher pressure from working with thebias to position the element of the pump pilot cylinder to a neutralposition.

FIG. 4 illustrates how FIGS. 5 and 6 are joined together to show acomplete control system for a jointly controlling two separatetransmissions 30R and 30L as would be employed in a track-type vehicle.In such a vehicle, steering is effected by driving one track at adifferent speed in relation to the other, and the control systemillustrated provides structures for such a steering capability.

For convenience, the parts of the basic control system and transmissionsin FIGS. 5 and 6 are designated the same numerals where identical asused to describe the embodiment shown in FIG. 1. The heart of thecontrol system shown in FIGS. 5 and 6 is very similar to that shown inFIG. 1, with the exception of the additional structures to provide thecontrol system with greater flexibility. Specifically, additionalstructures are included to provide for steering in track-type vehicles,braking of the which automatically, correcting overspeed and underspeedcontrol, override control for control convenience and other features.complemented by these additional structures, the basic control systembecomes a sophisticated system for controlling hydrostatic transmissionsin tracktype vehicles which is highly efiicient and effective.

Recapping briefly, the differential pressure control system shown inFIGS. 5 and 6 consists of a control pump 70 driven by the engine, aventuri 72, a differential pressure stack 74, a directional valve 96 andpump and motor pilot cylinders '90 and 100, respectively. The aboveelements are associated in a manner as previously described and functionsimilarly to the system described in FIG. 1, except insofar as theoperation of the basic system is effected by the additional componentsshown. For convienience, the additional components will be discussedseparately and their purpose, along with their eflect on the operationof the basic system, will be note-d.

In the differential pressure stack 74, shown in FIG. 5, there is anadditional path between the tubes 75 and 76 controlled by the reliefoverride valve 130. Normally, the

llll spool 131 of this valve is biased to the closed position by spring132 so that it has no effect on the differential pressure within thestack.

The purpose of the override valve is two fold, in that it provides asafety feature if an overpressure condition develops in thetransmission, and also provides a flexible control for the operator.This latter function is accomplished through plunger 133 in the valvebody which is actuated by operator foot pedal 134 through linkages 135.By means of the pedal, the operator can effect a reduction of thepressure differential within the stack which has been previouslyestablished by setting the speed control lever 82. The spool of thisvalve has metering slots so that depression of the foot pedal willgradually slow the vehicle from the preselected speed range down to stopwithin this range as the operator depresses the pedal without using thespeed control lever. When the path through the spool is wide open, thedifferential pressure in the stack will be zero, notwithstanding thesetting speed control valve '82.

Cooperating with the relief and override valve 130 is a safety reliefcylinder 136 which has its plunger 137 spring-biased to a fixed positionwithin the cylinder body. Orientation of the plunger is such that itwill cause the override valve 130 to open if the safety relief cylinderis actuated by high pressure blow-oif from the hydrostatic loop. Thus,if an overpressure occurs within the hydrostatic loop, the safety reliefcylinder will automatically reduce the differential pressure within thestack, thereby reducing the fluid flow within the hydrostatic loop. Thisprevents the continuation of power flow through the hydrostatic looponce an overpressure has occurred and prevents a large amount of heatfrom being generated in the transmission relief system which couldresult in failure.

In the higher speeed ranges of the transmission there is usually notsufficient compression braking to slow the vehicle under some conditionsand engine overspeed may occur. For this reason it is desirable to havean overspeed control system 139, as shown in FIG. 5. This overspeedcontrol system also uses the differential pressures available in thebasic control system for actuation. Pressure differ ential developed bythe venturi 72 is used to actuate the vehicle brakes to retard it duringpower generation, such as will occur during vehicle deceleration ordownhill operation. During downhill traverse, for example, powergeneration may occur when the transmission is in the higher speed rangesto the extent that engine compression braking will not be able tocontrol vehicle speed. Under these circumstances, with the motor actingas a pump and the pump as a motor driving the engine, an engineoverspeed can occur. The overspeed cylinder 140 uses pressure balancingby connecting line 141 to the low pressure tap 72a of the venturi andreflecting it on one side of piston 142 in the overspeed cylinder. Onthe opposite side of the piston within the cylinder pressures upstreamof the venturi from line 71 are reflected through line 144 and operateagainst the combined bias of spring 145 and the lower pressure from theventuri tap which normally hold the piston bottomed at one end of thecylinder when no overspeed condition exists. The rod 142a of the pistonis connected through linkages 146 to the air brake boost unit 147 whichcontrols the application of the brakes.

During operation of the overspeed system, brake band 148 on brake drum149 is applied to slow the vehicle during overspeed conditions becausethe pressure in line 141 will decrease due to increased flow through theventuri because of the higher r.p.m. of the control pump 70, andtherefore, the pressure reflected through line 144 will cause the systemto actuate the air control unit for retarding the vehicle until theoverspeed condition is terminated. Other kinds of control units may beemployed.

Another desirable function can be added to the basic control system byincorporating a low speed cut-out valve 155 serially with the underspeedvalve in the path between the tubes 75 and 76 of the differentialpressure stack 74. Basically, the cut-out valve is designed to give thecontrol system two modes of operation by disabling the underspeed val'veat low engine r.p.m. so that the vehicle can be moved with the engine ator near idle. Without this cut-out valve, the underspeed valve wouldsense an underspeed condition until the engine was operating near ratedrpm. and prevent the transmission from establishing power flow.Incorporation of this cut-out valve allows the transmission to beoperated at or near idle, but without an underspeed control feature.However, in this range underspeed is not aproblem.

The cut-out valve is very similar to the underspeed valve but operatesin a reverse mode since it closes the parallel path at lower enginespeed but remains open at higher engine r.p.m. where underspeed controlis necessary. During vehicle operation, low pressure from the lowpressure tap 72a of a venturi is reflected on a floating cut-out spool156 which is biased toward a closed position by spring 157 and thispressure. Reflected on the opposite side of the cut-out spool throughline 158 is the pressure from the control pump which above certainselected engine speeds will position the spool to open a flow paththrough the valve. However, when engine speed is in the area of idle,the pessures from the low pressure tap are higher and, augmented by thespring bias, will close the cut-out valve. In this condition, the speedcontrol valve can be used to create a differential pressure within thestack since the underspeed valve is rendered inoperable even though theengine rpm. is not at rated.

As noted above, in dual transmissions for track-type vehicles as shownin FIG. 6, means must be provided in the basic control system toindependently control the indvidual transmissions for steering thevehicle. Control of the motors in dual transmission systems isrelatively easy with the control system of this invention. Basically,the servo control arms 44 of each motor are slaved together throughlinkages and controlled by a single motor pilot cylinder so they reactin unison. In FIG. 6, special scissor linkage is used to slave the servocontrol arms of the motors with one another. This linkage has two bellcranks 151 which are pivoted on appropriate supports and linked to therod 104]; of the piston in the motor pilot cylinder so that the bellcranks move together. By this arrangement, the movement of the rod willsimultaneously move the servo control arms of the motors an equal amountso equality of track speed on opposite sides of the vehicle will alwayshe maintained so long as fluid input to each motor is the same.Adjustable linkages 152 are used to connect the bell crank and the servoarms in the linkage to allow independent adjustment of the motors sothat motor synchronization can be accomplished.

In an embodiment of the control system having a steering capability, asshown in FIG. 6, to control dual transmissions, a single pump pilotcylinder 90 may be employed; and when steering is not being employed,its operation is identical to that described with reference to a singletransmission system. However, to incorporate a steering function, it isnecessary to connect the pump pilot cylinder to the servo control arms41 of the pumps through special linkages so that each transmission canbe independently controlled or operated jointly. Through the steeringlinkage 160 shown complementing the basic control system in FIGS. 5 and6, a greater versatility in steering track-type vehicles is providedsince it allows one track to be reversed in a direction relative to theother track while at all times maintaining the tracks in a poweredcondition.

Specifically, the special steering linkage 160 includes two steeringcylinders 161 each mounted on a pivoted base plate 162. A base plate islocated close to each of the pumps in the dual transmission system, ascan be seen in FIG. 6, and swings about a fixed pivot 163. A protrudinglug 164 on each base plate is coupled to a tie rod 165 which spansbetween the transmissions and ties the lugs together so that the baseplates will be swung about their respective pivots in unison. In turn,the tie rod is connected to the pump pilot cylinder 90 which through thetie rod, swings the base plates about their respective pivots in unison.Tie rod 165 includes an adjusting nut 165a by which the respective baseplates may be adjusted relative to one another.

Mounted on each base plate is a steering cylinder whose piston rod 166is oriented to reciprocate across the axis of the pivot 163 of itsassociated base plate. The travel of the rod is usually restricted to anequal distance on each side of the pivot axis and each rod connected toa piston 167 within its associated steering cylinder. Each piston inturn is held at one end of the cylinder with a spring 168, and theoutboard end of its associated rod is connected through a linkage 170 tothe servo control arm 41 of the adjacent pump. An adjustment 170a onthis linkage is used to adjust the linkage to the pump has zerodisplacement when the connection between the linkage and the end of therod is directly over the pivot axis of its base plate.

Once preset to this condition and extended by springs 168, the baseplates are swung about their pivots to change the pump displacement ofboth transmissions an equal amount. Subsequent movement of the rod 166by cylinder actuation to a point directly over the pivot axis willchange the displacement of that pump to zero and further movement of therod (in the same direction) will carry the connection to the oppositeside of the pivot, which increases the displacement of the pump to itsprior displacement but with a power flow in a reverse direction. Itshould be appreciated as the connection of the rod and linkage moves,the base plate remains stationary so that the transmission on theopposite side of the vehicle remains unchanged. The geometry of thelinkage can be varied to obtain the correct degree of movement for theservo arms.

Using this arrangement, it is possible to set both transmissions at anequal speed for either a forward or reverse direction through controlactuation of the pump pilot cylinder and thereafter by actuation of thesteering cylinder associated with one of the transmissions toindependently slow that transmission and reverse it to the same speedbut in the opposite direction. By increasing the fluid pressure in asteering cylinder sufliciently the requisite movement of the rod can beeffected for such steering operations. The specific linkage described isvery desirable since it operates in either a forward or reversedirection without the necessity of complicated reversing valves or thelike.

The full steering linkage is illustrated in FIG. wherein a bridge valve175 is shown for controlling the pressure flow to the individualsteering cylinders. The bridge valve is really two valves in one whichhas a pair of identical spools joined endwise in the single spool 176.

,The spool is spring-centered by springs 177 at one end and can beattached to various means for operating it at the other.

The bridge valve has a central port which is connected to a source ofpressure fluid, such as line 40. Fluid pressure entering the valve isrouted to a groove 179 and confined by a land on the spool 176 when itis centered by its centering springs. Movement of the valve to the rightor left first brings one or the other set of small flow orifices 180into communication with the pressure fluid so that it begins to flowfrom the central groove toward one or the other drain grooves 181.Further movement of the spool in the same direction will bring largerslots 182 into communication with the central groove so that the flowwill be increased. As this is occurring another portion of the valvespool is simultaneously closing off the drain groove with similarpassages and slots a and 182a. This causes pressure to build graduallyin one of the ports 183 between the central groove and one of the draingrooves. These ports are connected to opposite steering cylinders 161through lines 184.

The above bridge system is schematically displayed in FIG. 11, and FIG.12 is a pressure profile developed by the use of the orifice and bleedslots arrangement described above. It can be seen from the pressureprofile 185 that the initial movement of the valve builds the pressureup rather sharply to a steering point and thereafter continued movementof the spool gradually increases the steering pressure up to the pointthat the travel of the rod of one of the steering cylinders begins toreverse the power flow in its associated pump. This is the point wherethe reversal of tracks begins. In this manner, excellent steeringresponse is achieved with a simple, economical system which provides amaximum flexibility. The spool in the bridge valve may be controlled byany suitable meahs such as a wheel, lever, etc.

A simple steering valve 175 is shown in FIG. 5 and actuated by a lever186 connected to its spool. Lines 184 connecting the valve to thesteering cylinders include a parallel ball check and orifice unit 188which smooth out the steering if desired.

In FIG. 11, the steering circuit just described is schematicallyillustrated. In this particular embodiment, the ports and slots 180,180a, 182 and 182a must provide the pressure profile as shown in FIG.12.

In FIG. 7, dual transmissions in a track-type vehicle 190 areillustrated for a better understanding of the above descriptions.Transmissions 30R and 30L are mounted in the vehicle in a side-by-siderelationship and some of the actuators and other control hardware arealso shown by way of illustration. The pumps 31 of the transmissions aredriven from a common driveshaft 192 by a bull gear 193 and gears 194.This arrangement allows power regeneration to be interchanged betweenthe transmissions through the common bull gear. Further, no clutch isnecessary since the displacement of the pump in each transmission can beadjusted to zero and power flow in the transmission stopped. Also, sincethe novel control system is controlled by fluid pressures, the operatorstation can be located /remote from the transmission and control system.

Since the transmissions discussed above do not have a clutch betweentheir pumps and the engine, there is a possibility that the vehicle maybe started in gear, i.e., with the speed control lever at go position.Thus, if the operator starts the engine, the vehicle may undergounexpected movement, creating a safety hazard. For this reason, it isdesirable to incorporate safety features in the control system whichprevent unwanted movement on start up. In FIG. 8, an improved relief andreplenishing group 47a is shown in a single housing 200 which providesstart up safety features, as well as the dual functioning elements whichare an improvement over the prior art group shown in FIG. 2.

The replenishing section 201 of the housing 200 includes a cylindricalchamber 202 which has communication with pressurized line 40 and isclosed by poppet spools 203 at opposite ends. Each poppet spool has astepped head 203a arranged so that the stepped surface 203!) hascommunication with an annular recess 204 in the housing which, in turn,communicates through port 205 with one of the conduits in thehydrostatic group, so that one annular recess at one end of the chamberhas communication with conduit 33 while the other recess communicateswith conduit 34.

Springs 206 urge the heads of the poppet spools into chamber 202 wherethey seat to close the chamber. The bias of the springs is such that thepressurized fluid in line 40 will overcome it, depressing the poppetspools 15 and allow fluid to flow into the hydrostatic loop to replenishit with cool hydraulic fluid.

During power flow in the transmission, one of the conduits will beoperating at a high pressure in order to transmit power from the pump tothe motor, and this higher pressure is reflected via port 205, recess204 and through orifice 207 into a balance chamber 208 behind the poppetspool which will allow pressures to equalize so the poppet spool canmove out of the balance chamber under the influence of the springclosing off flow through the replenishing section from the high pressureside of the loop.

In the replenishing section shown in FIG. 8, the balance chambers arevented to reservoir through a channel 208a which is closed by a ballvalve 209. It can be appreciated that unless the ball valve is seated,it will not be possible to operate the transmission since the fluid canbypass the motor through the poppets as described above. The ball valveis seated by directing pressurized fluid via line 213 to plunger 214whose stem portions bear on the ball valves causing them to seat andclose the vent to reservoir. Ball valve 209 may be replaced with othervalve types.

In turn, the safety-reset valve controls the flow of pressurized fluidto the plungers and prevents the transmission from being actuated untilthe operator eflects a manipulation in the safety system.

The safety-reset valve 220 is shown in FIG. 9 and when the transmissionis reset pressurizes line 213 through port 221 so that the plungers willclose the ball valves in the replenishing section of housing 200,thereby activating the transmission.

If the park lever (not shown) is released to the 011 position, link 222will release spool 223 in the housing allowing spring 224 to urge thespool to a position which will open a flow path from pressurized line 40through port 225 into port 226 where the pressurized fluid can pass viaball check 227 to the reset spool 228. The reset spool in turn controlsthe passage of the pressurized fluid to port 221 from which it iscarried to the plungers in the replenishing group for activating thetransmission.

Construction of the reset spool is such that land 229 closes off port221 until the spool is moved into spring 230 sufiiciently to establishfluid communication between the ball check and this port. When thetransmission is disabled, the reset spool will have the position shownin FIG. 9 so that pressurized fluid passing the ball check can, throughorifice 231, pass into chamber 232 by land 233. A bleed port 234 in thespool bleeds chamber 232 to reservoir when the spool is in the safetyposition, as can be seen in the drawing. In operation, after the parklever is released and spool 223 opens a flow path of pressurized fluidto ball check 227, the reset spool can be pushed into its spring whichwill cause passage 234 to be closed by the valve body and allow fluidpressure passing the ball check and acting on land 233 to hold the resetspool against the spring bias. In this position, it allows thepressurized fluid to pass into the port 221. The reset spool isgenerally located adjacent to the speed control lever and oriented sothat when the speed control lever is returned to the stop position, itwill depress the spool which will be then held by fluid pressure in theon position, providing the park lever is released.

To prevent hydraulically holding the reset spool in the activatedposition once set through a hydraulic lock, chamber 232 is bled throughport 235 via metering valve 236 to reservoir, which will allow the fluidtrapped in chamber 232 to bleed slowly to reservoir, thereby allowingthe reset spool to move under the spring pressure to open the chamber toreservoir and disable the transmission when fluid flow to the spoolfalls below the quantity being bled from the chamber. This wouldnormally occur on shut down or placement of the park lever in the onposition. In this way, the transmission is disabled until the park leveris released and the spood control lever is returned to stop. Thisprovides a very desirable safety feature for hydrostatic transmissionsand this safety fea ture will eliminate some of the hazards that will bepres out due to the operators unfamiliarity with a hydrostatic type oftransmission system in track-type vehicles.

The vent and overpressure relief functions are also combined in thehousing 200 shown in FIG. 8 in the vent overpressure section 240. Thissection has a master chamber 241 which is divided into two secondarychambers by a shuttle piston 242 which is free to reciprocate betweenthe chambers, but prevents fluid communication therebetween. One of thesecondary chambers 241a is connected through its port 243 to conduit 33and the other secondary chamber through its port 233 is connected toconduit 34. Located at each end of the master chamber are poppet valves246 which close ports 247 at opposite ends of the master chambertherefrom. These ports lead via passage 248 to reservoir. Each poppetvalve is biased by a spring 24? to close the communication between itsassociated port and secondary chamber. Behind each poppet valve is abalance chamber 250 which drains to reservoir via port 251, which inturn is closed by a spring-loaded poppet 252. Each poppet valve has anorifice 253 in its face so that the pressure in the secondary chamberscan pass through the orifice to the balance chamber to equalizepressures.

Utilizing the above construction, when the transmission is activated,the vent and overpressure valve will function in the following manner.The pressure from either conduit 33 or 34, the one operated at thehigher pressure, will enter one of the secondary chambers 241a. throughport 243 and cause the shuttle spool to move into the I oppositesecondary chamber. As this occurs, a nose piece 2420. on the end of theshuttle piston will engage the face of the poppet valve 246 depressingit and open the lower pressure conduit through ports 243 and 247 and viacoolers and relief valves to reservoir so that additional fluid will beremoved from the low pressure side of the hydrostatic loop so additionalfluid can be added for cooling. The opposite poppet then becomes thehigh pressure relief overpressure valve since high pressure fluid in itssecondary chamber may enter the balance chamber behind its poppet valveand, with the assistance of the spring, hold this poppet valve closed aslong as the pressure in the balance chamber does not exceed the springbias on the small poppet 252. Should the latter occur, high pressurefluid will be vented from the balance chamber into ports 254 to the highpressure override cylinder 126 (see FIG. 5) via line 255 and the poppetvalve will dump to the opposite conduit (33 or 34) through passage 248.

During the above description of the basic control system, mention wasmade of parallel ball check and orifice units 97 which were used tosmooth out the operation of the control system so that abrupt changes insetting would not be accompanied with violent transmission response.While the parallel ball and orifice units do smooth the transmissionresponse, the rate of response varies in proportion to the differentialpressure established in the stack 74. For this reason, it is desirableto use a variable orifice device 270, shown in FIG. 13. The variableorifice device includes a housing 271 having a bore 272 therein.Reciprocably mounted within this bore is a spool 273 that has a land atboth ends and a reduced diameter center portion. In the area of reducedcenter portion, a plurality of radial orifices 274 is drilled ingenerally axial alignment so that as the spool moves within the boreagainst the bias of spring 275, one of its lands 273a will seriallyclose the orifices and thereby restrict fluid flow from the bore tochamber 276. Fluid is supplied to the bore between the lands of thespool via an axial passage 277 in the spool which extends from one endthereof and vents into bore 272 between the lands. By this arrangement,fluid pressure entering the unit via port 278 can pass through the axialpassages and thence through the orifices to chamber 276. Spring 275biasing the spool 273 to keep the orifices open 17 is located in abalance chamber whose port 279 is in communication with tube 76 of thedifferential pressure stack.

Utilizing this unit, it is possible to give the control system a smoothresponse over its full range independent of the differential pressuredeveloped in the stack.

By way of illustration, reference is made to FIG. 1 wherein the ballcheck and parallel orifice device 97 may be disconnected from line 101and this line in turn connected to chamber 276 with port 278 connecteddirectly to tube 75 of the differential pressure stack. Connected inthis manner, the greater the pressure differential in port 278 to thatin port 279, the more the spool will move into spring 275, closing offmore and more of the orifices 274. Thus, at higher differentialpressures, fewer orifices will be available for the transmission ofhydraulic fluid to the pump pilot cylinder 90. Conversely at lowerdifferential pressures more of the orifice passages will be exposed andresponse will tend to be uniform throughout the control range. As thedifferential pressure is reduced and reverse flow through the unitoccurs, spring 275 will displace the spool so that all the orifices areopen from chamber 276 to port 278, so that in elfect it operates likethe check valve.

Utilizing these difierential orifice devices which, like many of thecomponents of the control system, are based on difierential pressureprinciples, it is possible to give the basic control system smoothresponse throughout its entire control range so that operator comfortand safety will be maintained. Also, it should be appreciated when thevariable orifice units are connected in the control system, port 27 8will be in communication with either the high pressure tube 75 of thedilferential pressure stack, or tubes 95 and 102 of the pump pilotcylinder and motor pilot cylinder, respectively. This can be appreciatedby noting the orientation of the ball and check device 97 in FIG. 1 forwhich variable orifice units 270 are substituted in the refined versionof the control system or other means.

An alternative embodiment of the dilferential orifice device can befabricated by locating port 276 so that it has direct communication withbore 272 in the area of the reduced portion of the spool 273 anddeleting the axial passage 277 in the spool. In this embodiment one endof the spool is connected with one side of the pilot cylinder and theopposite end of the spool is connected with the other side of the pilotcylinder being modulated. Connected in this manner the unit providesadditional modulation since the fluid passing to or from the pilotcylinder through port 278 is modulated in proportion to the pressures onthe opposite sides of the pilot cylinder.

In the drawings, R is used to refere to a common reservoir.

What is claimed is: 1. A steering control system for controlling pairedhydrostatic transmissions in crawler-type vehicles and the like whereinsaid transmissions are driven by a common prime mover and independentlycontrolled for steering by changing the displacement of the variabledisplacement pump including a control servo in each transmissioncomprising:

pivoted support means located adjacent to each hydrostatic transmission;

a tie rod between said support means coupling them for simultaneousswingable movement about their respective pivots;

a steering cylinder having a reciprocating element fixedly mounted oneach said support means so said element reciprocates across the pivotaxis of its associated support;

links connecting each said element to its associated transmission pumpcontrol servo so that movement of said support means will simultaneouslyadjust the servos of both said transmissions and independent movement ofeach element will independently control its associated transmission; and

actuating means for independently controlling each steering cylinder forsteering by changing the speed of one transmission relative to theother.

2. A steering control system as defined in claim 1 wherein the actuatingmeans for controlling the cylinder means includes a valve means fordirecting fluid pressure to the steering cylinder to efiect a change inthe position of their reciprocating elements and spring means biasingsaid elements to a fixed position when fluid pressure is not appliedthereto through said valve means.

3. The steering control system as defined in claim 2 wherein said valvemeans includes variable flow passages so that steering reaction has aprescribed relationship to the operators actuation of said valve means.

References Cited UNITED STATES PATENTS LEO FRIAGLIA, Primary Examiner I.A. PEKAR, Assistant Examiner US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,526,288 Dated Segtember l, 1970 Inventor(s) J. R. Cryder et a1 It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

AIR SUPPLY MB/ /FI :40 1 A 70! fin $8 84 r '1 e: 8 l 144 lss |f F76 7/ 1mam: 1 J 4 n flu! L II II E E FAST 79 E? SLOW 82 I HIGH vnessun: 7

83 255mm wuvrs Y J H/R Be :84 I35 I34 Bigncd and Scaled thisTwenty-seventh Day of June I978 [SEAL] Arrest:

DONALD W. BANNER RUTH C. MASON Attesting Oflicer Commissioner of Parentsand Trademarks

